The present invention relates generally to semiconductor devices, and more particularly to output buffers for semiconductor devices.
An important aspect of semiconductor devices is the switching speed of the output drivers. The switching speed of the output drivers are a major factor in the rate at which data can be transferred out of the device, and thus its operating frequency. Programmable logic devices are one type of semiconductor device that use output buffers, although most--if not all--semiconductor devices use output buffers, also.
An output predriver may be designed to adjust the pull-up and pull-down speed of the output buffer thereby altering the switching speed of the output driver. FIG. 1 illustrates a typical output buffer 100 with an output predriver such as may be included in currently available semiconductor devices. Output buffer 100 has a pull-up predriver 110, a pull-down predriver 120 and an output driver 130. Output driver 130 is further comprised of a pull-up transistor 134 and a pull-down transistor 138.
Both output predrivers 110 and 120 comprise a series of inverting buffers 142-146. The inverting buffers of pull-up predriver 110 may be appropriately sized to favorably adjust the switching speed for the low-to-high transition of pull-up driver 134. Similarly, the inverting buffers of pull-down predriver 120 may be sized to favorably adjust the switching speed for the high-to-low transition of pull-down transistor 138.
A disadvantage associated with output buffer 100 is that when the inverting buffers are sized to favor both switching from high-to-low and from low-to high in terms of speed, the design may cause a "crowbar current." A crowbar current is a short circuit current caused by both the pull-up driver and the pull-down drivers being on concurrently. This occurs because when the predrivers are advantageously sized to turn on the pull-up or pull-down drivers as fast as possible, the drivers are more difficult to turn off quickly. Thus, when either driver is turning on, there is a period of time when the opposing driver is still on due to predriver's inability to switch the opposing driver into the off state. This causes a crowbar current. The crowbar current is effectively analogous to a short circuit from power to ground through the concurrently "on" output transistors. Some detrimental effects of such a crowbar current include increased power bus noise, increased power consumption for active devices and, in some cases, non-optimal output switching speed.